1. Field of the Invention
The present invention relates to apparatus for reading a recorded medium in general and to an apparatus comprising a laser having an extended cavity which includes a recorded medium for reading the medium in particular.
2. Description of Prior Art
A variety of apparatus comprising a laser for reading data from a recorded medium has been proposed and many are used. For example, magnetooptic laser readout devices have been designed to operate by either the Kerr or Faraday effects. Both types require the detection of a small change in the polarization characteristic of polarized light that interacts with magnetic domains of a recorded magnetic medium. In the case of the Kerr effect, a polarization change is detected in light that is reflected from a magnetic surface. For the Faraday effect, the light is passed through a transparent or semitransparent magnetic material in which, again, the polarization angle of the wavefront is changed based on the orientation of magnetic domains.
In another prior known laser reading apparatus, changes in the reflectivity of the surface of a recorded medium, which changes correspond to data recorded on the medium, are detected using a semiconductor laser both as a light source and as a detector.
Enclosed herewith is a set of drawings in which FIGS. 1 and 2 are block diagrams of embodiments of the prior known apparatus described above for reading a recorded disk using laser light. In FIG. 1 there is shown a prior known Kerr effect magnetooptic reading apparatus. In FIG. 2 there is shown a prior known reflectivity responsive reading apparatus.
Referring to FIG. 1, there is provided in the prior known Kerr effect magnetooptic reading apparatus a source of laser light 101, a collimating lens 102, a beam splitter 103, a focusing lens 104, an apparatus for positioning the lens 104 in an R-Z plane as shown by crossed arrows 105 on a magnetic medium comprising an active magnetic layer 106 on a disk 107 having a transparent overcoat 108 and an apparatus for rotating the disk 107 as shown by the arrow 109, and a polarization analyzer system 120. The system 120 comprises a polarizing beam splitter 111, a pair of lenses 112 and 114, a pair of light detectors 113 and 115 and a differential amplifier 116.
In operation, laser light that is plane polarized to a very high degree is collimated at lens 102, passes through the beam spltter at 103 and is then positioned and focused by lens 104, by means of the movement system 105, onto the active layer 106 of the disk 107. The laser light is then reflected from the layer 106. The reflected light proceeds back through the optics 104 and 103 where it is directed to the polarization analyzer system 120. At the analyzer system 120, the polarizing beam splitter 111 separates the polarization components of the reflected light after which the lenses 112 and 114 focus the light on detectors 113 and 115 respectively. The outputs of the detectors then become the inputs to a differential amplifier 116 whose output will thus change with a change of polarization rotation at the surface 106.
Although the laser source 101 could be made very small, the optics and the focusing mechanism of the above-described prior known magnetooptic readout apparatus is very large. Furthermore, the limits imposed by diffraction in the lenses limit the maximum bit density that can be resolved, and therefore read back.
Referring to FIG. 2, there is provided in the prior known disk reflectivity responsive reading apparatus a semiconductor laser diode 201, an adjustable laser bias current circuit 215 comprising a battery 208 and a variable resistor 209, a capacitor 210, a lead 206 for coupling to a detector of laser voltage, a pair of lenses 202 and 203, a photodetector 204 for receiving light from a rear facet of the diode 201, a phase randomizing plate 211, and a recorded medium 207 comprising reflective and non-reflective areas corresponding to data stored thereon.
In operation, light from the laser diode 201, with bias current provided by bias circuit 215, is collimated at lens 202, phase randomized at plate 211, and focused with lens 203 on the recorded medium 207. Changes in the magnitude of the light reflected from the recorded medium are detected by either a change in laser voltage at lead 206, or by a change in laser light from the rear facet of the laser 201 at the photodetector 204.
While the last described apparatus has the advantage of using the laser as both the light source and a detector, it still suffers from the problems of focusing and the diffraction limitations described above with respect to the embodiment of FIG. 1.
In general, it is important to note that in none of the above-described prior known laser reading apparatus is the recorded medium an integral part or component of the laser optical cavity.
An apparatus in which an external reflective member is an integral part or component of the laser optical cavity has been disclosed by R. O. Miles, A. Dandridge, A. B. Tveten and T. G. Giallorenti in their article "An External Cavity Diode Laser Sensor", which appeared in the Journal of Lightwave Technology, Vol. LT-1, No. 1, March 1983. In this apparatus, forming a semiconductor laser sensor, a front facet of a diode laser is arranged to be held within a few wave lengths of a reflective member. As the reflective member is perturbed, even slightly, the phase of the light reflected back into the laser cavity is altered thus varying the effective laser facet reflectivity. The change in laser output is detected as a measure of the distance the reflective member moves relative to the exit facet of the laser.
While the article describes an external or extended cavity laser in which a reflective member is an integral part of the laser cavity, the description is restricted to an apparatus responsive to phase changes in reflected light in the optical cavity. There is no disclosure or suggestion of any use of such a laser for reading data from a recorded medium of any kind.